THERMO CHEMISTRY. All chemi cal reactions are accompanied by changes in temperature. Exactly when this fact was first observed, it is impossible to say, but it must have been in the very early years of chemical investigation, for the phenomenon is so marked that it could not long escape notice. In the majority of cases, there is a rise in tem perature, or, in other words, heat is evolved, but in some cases, the tempera ture falls, or heat is absorbed. Reac tions of the former type are known as exothermic, those of the latter as endo thermic. The first law governing these thermal changes was formulated by La voisier and Laplace, who stated, as the result of their investigations, that: The amount of heat which is required to de compose a compound into its constituents is exactly equal to that which was evolved when the compound was formed from these constituents. A second law of even greater importance was discov ered by Hess, who demonstrated that: The heat evolved in a chemical reaction is the same whether it takes place in one or in several stages. For instance, car bon may be burned either to carbon mon oxide or carbon dioxide, according to conditions. Carbon monoxide may be burned to carbon dioxide, so that in ei ther case the final chemical change is from carbon to carbon dioxide. The above law (known as the "Constancy of the heat sum") is, as a matter of fact, a direct consequence of a wider law—the principle of the conservation of energy— but it was worked out by Hess experi mentally. Of recent years important progress has been made in this branch of science as the result of researches carried out by Julius Thomsen of Copen hagen, and Berthelot, the famous French chemist. Thomsen devised new appara tus for making thermo-chemical determi nations, and collected a mass of thermo chemical data whose value can scarcely be over-estimated. Berthelot also made improvements in apparatus, and his dis coveries have resulted in a great ad vance in elucidating the theories under lying thermo-chemical phenomena. Three
of the fundamental principles which he laid down are: (1) The thermal change due to a chemical reaction depends (providing no external work is done) only on the con dition of the system at the beginning and end of the reaction, and not on the inter mediate conditions.
(2) The heat evolved in a chemical re action is a measure of the corresponding physical and chemical work.
(3) Every chemical change which oc curs without the addition of external en ergy tends to produce that substance or system of substances the formation of which is accompanied by the evolu tion of the maximum amount of heat. This generalization is found to be not true in every case, but it applies to so many instances that it is generally held to indicate some natural law which is not, at present, fully understood.
The Heat of Formation of a substance is the amount of heat liberated or ab sorbed when the substance is produced by a direct combination of the elements of which it is composed. It is customary to determine the heat of formation on gram-molecular weights; so that, quan titatively, the heat of formation of any substance is expressed as the amount of heat liberated or absorbed when a gram molecular weight of the substance is formed by combination of its elements. In those cases where compounds cannot be formed from direct combination of elements, it is necessary to determine the heat of formation indirectly. This is commonly done by first burning the elements in oxygen, then burning the compound in oxygen, and determining in each case the heat liberated. The prod ucts of combustion will, in each case, be the same, so that the difference in the heat liberated in the two cases will give the heat of formation of the compound.